Reverberation chamber loading

ABSTRACT

A method and system for selectively varying the performance of a test chamber are disclosed. According to one aspect, the performance is affected by a variable absorbing structure of the test chamber. The absorbing structure enables selective exposure of absorbing material to achieve a specific performance.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of U.S. Provisional Application No.61/823,604, filed on May 15, 2013, entitled “REVERBERATION CHAMBERLOADING” and which is incorporated herein in its entirety by reference.

FIELD

The present description relates to a method and system for variable,controllable, and repeatable loading of a reverberation chamber.

BACKGROUND

Reverberation chambers are used for generating statistically uniform andisotropic distributions of electromagnetic or acoustic energy within atest volume. In an electromagnetic reverberation chamber, the walls,ceiling and floor are generally metallic and highly reflective ofelectromagnetic energy. In an acoustic reverberation chamber, the walls,ceiling and floor are of a material such as concrete that is highlyreflective to acoustic waves. The distribution of energy within areverberation chamber is multimodal, with energy being transferredbetween modes as a consequence of reflective baffles or shaping of theroom itself. Energy within the reverberation chamber may be stirred inthree dimensions by a variety of methods including rotating reflectivepaddles or baffles, so that the stirred energy dominates the fields inthe test volume. Also, variations can be obtained by rotating or movingthe signal generating source or measuring device within the volume ofthe chamber.

FIG. 1 is a diagram of a typical electro-magnetic reverberation chamber10 having a metallic floor 12, metallic ceiling 14, and four metallicsides 16 (only two sides shown) to enclose a device under test, DUT. InFIG. 1, a measurement antenna 20 may be mounted on a first positioner21, and a calibration antenna 22 may be mounted on a second positioner23. The positioners may be controlled by software and/or a user.

A line of sight, LOS, shield 24 may be mounted between the measurementantenna 20 and the calibration antenna 22. The LOS shield 24 may bemetallic and used to block line of sight electric field componentsbetween the two antennas 20 and 22.

The reverberation chamber 10 may also include a horizontal z-fold tuner26 and a vertical z-fold tuner 28. The z-fold tuners 26 and 28 may bemade of large aluminum reflecting sheets supported on either a rigid boxframe or a single spine and designed to provide the efficient reflectingsurfaces desirable for use in the reverberation chamber 10. Inparticular, at one end of the vertical z-fold tuner 28 is pulley wheel30 which rotates antenna 20 at a fixed ratio with respect to thevertical z-fold tuner 28.

SUMMARY

Embodiments described herein advantageously provide methods and systemsfor affecting the performance of a test chamber. According to oneaspect, the performance is affected by a variable absorbing structure ofthe test chamber. The absorbing structure enables selective exposure ofabsorbing material to achieve a specific performance.

In one embodiment, the test chamber includes a reflective material andan absorbing material. The absorbing material is at least partiallyshielded by the reflective material. The reflective material isadjustable to expose at least a portion of the absorbing material.

One embodiment is a method of affecting performance of a reverberationchamber. The method includes positioning a variable absorbing structurewithin the reverberation chamber at a predetermined position andorientation to achieve a repeatable specific performance.

Another embodiment is a reverberation chamber that includes an enclosedhousing and an absorbing structure having absorbing material. Theenclosed housing has at least one at least partially reflective interiorwall. The absorbing structure is configured to selectively expose theabsorbing material to achieve a repeatable specific performance.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is an illustration of an known electromagnetic reverberationchamber;

FIG. 2 is an illustration of rays arriving at one point within areverberation chamber from some other point within the reverberationchamber;

FIG. 3 is an illustration of a variable absorbing structure constructedin accordance with principles of the present invention;

FIG. 4 is an illustration of a variable absorbing structure constructedin accordance with principles of the present invention;

FIG. 5 is an illustration of a variable absorbing structure constructedin accordance with principles of the present invention;

FIG. 6 is an illustration of a variable absorbing structure constructedin accordance with principles of the present invention;

FIG. 7 is an illustration of a variable absorbing structure constructedin accordance with principles of the present invention;

FIG. 8 is an illustration of rays in an elongated reverberation chamber;

FIG. 9 is an illustration of rays in a reverberation chamber with floorand ceiling treated to reduce rays emanating at or near right angles tothe floor and ceiling;

FIG. 10 is an illustration of a plan view of two and three dimensionalstirrers; and

FIG. 11 is an illustration of an elevation view of two and threedimensional stirrers.

DETAILED DESCRIPTION

Before describing in detail exemplary embodiments that are in accordancewith the present invention, it is noted that the embodiments resideprimarily in combinations of apparatus components and processing stepsrelated to variable loading of a test chamber. Accordingly, the systemand method components have been represented where appropriate byconventional symbols in the drawings, showing only those specificdetails that are pertinent to understanding the embodiments of thepresent invention so as not to obscure the disclosure with details thatwill be readily apparent to those of ordinary skill in the art havingthe benefit of the description herein.

As used herein, relational terms, such as “first” and “second,” “top”and “bottom,” and the like, may be used solely to distinguish one entityor element from another entity or element without necessarily requiringor implying any physical or logical relationship or order between suchentities or elements.

Reverberation chambers have a wide variety of applications, and areconventionally designed with the highest Q possible. When the stirredenergy dominates the fields in the test volume, the temporal signatureof the transmitted waves is extremely long, resulting in a highlyreverberant environment. However, in some applications, the statisticaluniformity and multipath nature of the reverberant environment aredesirable, but the high Q does not provide a realistic test environmentto simulate desired real-world behaviors.

For example, reflections produced in a reverberation chamber are assumedto be statistically uniform in both spatial and magnitude distribution.While the chamber dimensions, by design, are not identical in eachdirection, the loss of the surface of the cell is low enough that theprobability distribution of a stirred cell can be assumed to bespherically uniform within the test volume. Such a distribution isillustrated in FIG. 2. FIG. 2 is an idealized illustration of line ofsight propagation and propagation of a first few reflections of a signalpropagating between two points within the chamber.

In many cases it is desirable to selectively load a reverberationchamber to achieve a desired temporal behavior, for example, an RMSdelay spread. However, a device under test will load this reverberationchamber, thus making a single fixed loading impracticable for typicalapplications. Instead, methods and apparatus described herein forapplying variable loading to a reverberation chamber while stillmaintaining a desired uniformity and isotropy are desirable in manyapplications. For example, the chamber may be loaded in all directionsuniformly, or selectively loaded by absorber placed out of a line ofsight between an antenna and a device under test, in order to reduce theRMS delay spread while keeping a uniform distribution within a testvolume.

However, for terrestrial wireless communication, for example, thedistribution of received signals in a multipath environment tends tocluster near the horizon, largely due to the propagation distancesinvolved and the angles of incidence of direct and reflected rays withinthe environment. Since communication generally occurs between two pointsalong the horizon, specular reflections off of any surface, includingthe earth, structures, and even interior walls, ceiling, and floors, alltend to have angles of incidence with elevations that cluster near thehorizon, with little energy coming from directly above or below theradio. Only scattering reflections, usually from small and randomlyplaced objects (e.g. pipes, lights, etc.) are likely to produce anglesof arrival within these elevations, typically with a lower magnitude, aswell as probability, of those from a larger reflecting surface like theside of a building.

While the approaches described herein primarily focus on electromagneticapplications, the concepts, methods, and apparatus described hereinapply to acoustic reverberation chambers as well.

Selective loading of a reverberation chamber or cell invites carefulattention to placement and quantity of energy absorbing material.Desirably, selective loading of a reverberation chamber is repeatable,controllable and variable. Net energy loss within a reverberationchamber is primarily a function of surface area of the lossy materialused for loading relative to the overall size and surface area of thereverberation chamber itself. Since all of the walls, baffles, andstirring paddles of a reverberation chamber have loss, the maximum Q andmaximum K-factor—which is the ratio of stirred energy to unstirredenergy—of the chamber is affected by the loss of these components.

A theoretical model for a reverberation chamber may include a randomcollection of plane waves with the uniform spherical probabilitydistribution and the uniform magnitude and phase distribution. Such amodel implies that on average, when looking in any direction from thecenter of the test volume, the device under test will see a uniformillumination. The test volume is a sub-volume of the volume of the testchamber. Placing an absorbing surface in such a way that there is lineof sight visibility of that surface in the test volume, will result innon-uniformity of the spherical distribution and a reduction in thestirred energy. The reduction in the stirred energy may be proportionalto the solid angle subtended by the absorber divided by the solid angleof a sphere (4π), and the reflectivity of the absorbing material.Alternatively, placing the absorbing surface such that it is notdirectly visible from within the test volume may reduce the RMS delayspread without substantially affecting uniformity of the fielddistribution within the test volume.

The non-uniformity that may be achieved by selectively loading areverberation chamber may be desired in order to reproduce thenon-uniform distribution seen in specific real-world environments. Forexample, most radio propagation in the real world is limited to astatistical distribution that peaks near the horizon, and is made up ofcombinations of line of sight and reflected ray paths with relativelyshallow angles arising from ground bounce, ceilings and floors. Thus, itmay be desirable to modify the uniform distribution of a reverberationchamber to produce a spherical probability distribution that is Gaussianor Laplacian in theta and centers near theta=90°.

To simulate a specific environmental propagation condition,corresponding to losses associated with a certain distance orpropagation through walls or other lossy media, the rate of decay of thereflected signals within the reverberation chamber are desirably tunedby selective loading. By selectively loading the reverberation chamber,the RMS delay spread and overall power delay profile (PDP) can beadjusted. However, the initial RMS delay spread and PDP are a functionof the physical size and loss of the unloaded reverberation chamber, andthus, the amount of loading required to produce a particular behaviorvaries. Further, the object to be evaluated, when placed into the testvolume, affects the loading of the reverberation chamber as well. Thus,the additional loading necessary to adjust the RMS delay spread and PDPmay need to be varied after the device under test is placed within thechamber.

Rather than manually adding and removing different sized pieces ofabsorbing material—which may lead to inconsistent results—embodimentsdescribed herein provide one or more mechanisms to continuously,controllably and repeat-ably vary the amount of loading in thereverberation chamber. In some embodiments, one may repeat-ably andcontrollably reduce RMS delay spread without altering the uniformdistribution or the statistical nature of the reverberation chamber.Also, by selective loading as described herein, one may controllablyvary the homogeneity and uniformity of the fields in the test volume.One may also selectively reduce the energy of given polarizations anddirections within the test volume.

Returning now to the drawing figures, there is shown in FIG. 3 oneembodiment of a variable absorbing structure that is position-able in areverberation chamber to achieve repeatable, controllable, and variableloading of the reverberation chamber. FIG. 3 shows 3 differentconditions of the variable absorbing structure 32, 34, 36. Element 32 isthe variable absorbing structure in a fully open position. Element 34 isthe variable absorbing structure in a semi-open position. Element 36 isthe variable absorbing structure in a closed position. In the fully openposition 32, rectangular apertures 38 expose absorbing materialcontained within the variable absorbing structure, thereby loading thereverberation chamber with a specific amount of loss. In the partiallyopen position 34, the apertures 38 are partially blocked by a slidingreflective shield 40 that may be selectively adjusted to achieve adesired loading. In the fully closed position 36 the apertures 38 arecompletely blocked and no absorber is exposed. Note that the variableabsorbing structure of FIG. 3 enables repeatability in loading thereverberation chamber, by placing the sliding reflective shield 40 inthe same position and positioning the variable absorbing structure at asame position within the chamber. Note also that the loading caused bythe variable absorbing structure in the chamber is very controllable viaselective positioning of the sliding reflective shield 40.

Note that by controlling the placement of absorbing and reflectingsurfaces within the cavity, the embodiments of FIG. 3 may be employed toabsorb energy propagating within a first range of angles whilereflecting energy propagating within a second range of angles. That is,certain angles of propagation will be reflected while certain otherangles of propagation will be absorbed. In addition, the particularrange of angles for absorption and reflection may be selected byvariation of the location of the absorbing and reflecting components.This may be accomplished by covering or uncovering various portions ofan absorbing surface within a larger cavity. When the angles ofincidence through the apertures of the cavity align with the reflectingsurface, a low loss condition occurs, while when the angles of incidencethrough the apertures align with the absorber, a higher loss conditionoccurs. The range of angles and overall performance can be controlled bythe geometry of the apertures and the depth of the cavity. In additionto the mechanism used to alter the absorption angles, an additionalbaffle system can be used described above to vary the total amount ofloss of the directional absorbing system. These parameters may becontrolled to selectively restrict angles of propagation to a desiredrange of angles, while substantially suppressing propagation at otherangles. Note also, that instead of, or in addition to, a sliding shieldto variably expose the absorbing material, adjustable louvers may beemployed.

Thus, the orientation and aperture size of the embodiments of FIG. 3within the chamber may be employed to control the directivity ofabsorption, and consequently, the directivity of energy not absorbed.For example, the orientation of the apertures may cause selectiveabsorption of energy propagating in one direction while notsubstantially absorbing energy propagating in an orthogonal direction.The rectangular shape of the apertures also enables selectivity ofpolarization. For example, the orientation of the apertures may causeselective absorption of polarization in one direction while notsubstantially absorbing polarization in an orthogonal direction.

FIG. 4 shows another embodiment of a variable absorbing structure thatis positionable in a reverberation chamber to achieve repeatable,controllable, and variable loading of the reverberation chamber. Element42 is the variable absorbing structure in a first semi-open position.Element 44 is the variable absorbing structure in a second semi-openposition. Element 46 is the variable absorbing structure in a fullyclosed position. In the partially open position, the variable absorbingstructure exposes absorbing material 48 by sliding reflective shield 50.Note that the variable absorbing structure of FIG. 4 enablesrepeatability in loading the reverberation chamber, by placing thesliding reflective shield 50 in the same position and positioning thevariable absorbing structure of the same position within the chamber.Note also that the loading caused by the variable absorbing structure inthe chamber is very controllable by way of selective positioning of thesliding reflective shield 50.

FIG. 5 shows another embodiment of a variable absorbing structure thatis positionable in a reverberation chamber to achieve repeatable,controllable, and variable loading of the reverberation chamber. Element52 is the variable absorbing structure in a first semi-open position.Element 54 is the variable absorbing structure in a second semi-openposition. Element 56 is the variable absorbing structure in a fullyclosed position. In the partially open position, the variable absorbingstructure exposes absorbing material 58 by sliding reflective shield 60.Note that the variable absorbing structure of FIG. 5 enablesrepeatability in loading the reverberation chamber, by placing thesliding reflective shield 60 in the same position and positioning thevariable absorbing structure at the same position within the chamber.Note also that the loading caused by the variable absorbing structure inthe chamber is very controllable by way of selective positioning of thesliding reflective shield 60. Note further that a difference between theembodiment of FIG. 3 and the embodiment of FIG. 5 is the shape of theapertures exposing absorbing material. Shapes of the apertures otherthan rectangular and circular may also be employed and the shapes andsizes of the aperture may be selected to achieve a desired performance.

Note that any of the structures of FIGS. 3-5 may be built into one ormore walls and/or the ceiling and/or the floor of the reverberationchamber. Thus, for example, the ceiling of the reverberation chamber mayitself be constructed of a reflective material having variableapertures, as in FIG. 3, that expose absorbing material behind theapertures.

FIG. 6 shows an embodiment of a cylindrical variable absorbing structurethat is position-able in a reverberation chamber to achieve repeatable,controllable, and variable loading of the reverberation chamber. Element62 is the variable absorbing structure in a first semi-open position.Element 64 is the variable absorbing structure in a second semi-openposition. Element 66 is the variable absorbing structure in a fullyclosed position. In the partially open positions the variable absorbingstructure exposes absorbing material 68 by sliding reflective shield 70.Note that the variable absorbing structure of FIG. 5 enablesrepeatability in loading the reverberation chamber, by placing thesliding reflective shield 70 in the same position and positioning thevariable absorbing structure at the same position within the chamber.Note also that the loading caused by the variable absorbing structure inthe chamber is very controllable by way of selective positioning of thesliding reflective shield 70.

Note further, that that the sliding reflective shields 40, 50, 60 and 70may be driven by a motor that is remotely controlled by an actuatorlocated inside or outside the reverberation chamber. Also, the slidingreflective shields may be rolled about an axis when the variableabsorbing structure is fully or partially opened, thereby conservingspace.

FIG. 7 shows yet another embodiment of a variable absorbing structurethat is positionable in a reverberation chamber to achieve repeatable,controllable, and variable loading of the reverberation chamber. Thevariable absorbing structure of FIG. 7 is a box shielded on three sideswith absorber being exposed on the other three sides. As can be seenfrom FIG. 7, the variable absorbing structure can be placed in at leasteight different positions to achieve different loading of thereverberation chamber.

While randomly loading the cell with lossy material can produce an RMSdelay spread that is reasonable for wireless testing, random loading mayor may not significantly alter the uniformity of the field structurewithin the chamber. Nor is random loading necessarily repeatable. Incontrast, selective loading of the cell using the methods and apparatusdescribed herein, enables repeatable reduction of RMS delay spread,repeatable control of polarization, and repeatable control ofdirectivity, as well as repeatable control of other performanceparameters.

For example, through selective loading of the cell, and control of thevarious dimensions, it is possible to create a test environment wherethe energy is primarily constrained near the azimuth plane. Thus, it maybe desirable to modify the uniform distribution of a reverberationchamber to produce, for example, a spherical probability distributionthat is Gaussian or Laplacian in theta and centers near theta equal to90 degrees. The reverberation chamber modified in this way may alsobenefit from two dimensional stirring mechanisms that minimize theamount of energy directed into the elevation propagation directions fromthe azimuthal directions. As will be understood, the selectivedirectivity in other than azimuthal directions can be achieved applyingthe methods and apparatus described herein.

FIG. 8 shows an example of a reverberation chamber designed to alter thespatial field distribution to confine reflective rays to nearhorizontal. However, such a long and narrow chamber may be impracticalfor many applications. FIG. 9 shows that by selectively lining the topand bottom of a reverberation chamber with lossy material 72, raysbouncing off these surfaces can be weakened, thereby producing anelevation distribution clustered around the horizon. For example, thelossy material 72 may consist of absorbing material exposed by aperturesformed in the top and bottom of the chamber. Thus, by using absorber andreflector treatments that produce high loss at normal angles ofincidence, and that produce very low loss at shallow angles ofincidence, a gradually varying distribution can be achieved.

Moreover, by designing stir paddles that reflect energy primarily alongthe azimuth plane, 2-dimensional stirring can be achieved, minimizingthe amount of energy redirected to the absorbing boundaries. FIG. 10 isa plan view of a two dimensional stirrer 74 and a three dimensionalstirrer 76, and FIG. 11 is an elevation view of these two stirrers. Athree dimensional stirrer moves energy between all three orthogonalmodes arbitrarily with the objective of randomizing the components ineach mode to produce a uniform field. Conversely, the two dimensionalstirrer reflects energy between two orthogonal directions withoutreflecting energy into the third orthogonal direction. Note that thedefinition of two dimensional refers not to the geometry of the stirrer,but rather to the effect of the stirrer on the energy distribution.Thus, a paddle need not be restricted to two dimensions. For example, apaddle shaped like a plus sign (+) may be used as a two dimensionalstirrer. As another example, the two dimensional stirrer may be convex.Further note that the orientation of the two-dimensional stirrer isselected based on the desired range and orientation of angles to whichpropagation is to be limited. For example, a two dimensional stirrer maybe oriented and positioned to deflect rays at lower angles of incidenceto a more horizontal direction, thereby moving energy from an undesiredmode to a desired mode.

Thus, in some embodiments, a variable absorbing structure, with orwithout a two dimensional stirrer, is selectively positioned tosubstantially restrict propagation to elevations that are substantiallyhorizontal. In some embodiments, the variable absorbing structure isselectively positioned to substantially suppress propagation above aspecified elevation. More generally, in some embodiments, the variableabsorbing structure is selectively positioned to substantially suppresspropagation at one or more angles or a range of angles. The angle(s) orrange of angles selected to be suppressed may depend on the application.Alternatively, all but a selected range of angles may be suppressed byselective placement of absorbing structure and use of one or moretwo-dimensional stirrers.

Some embodiments described herein include a variable absorbing structureof a test chamber. In some embodiments, the structure includes areflective material and absorbing material at least partially shieldedby the reflective material. The structure is adjustable to selectivelyexpose at least a portion of the absorbing material. In someembodiments, the structure forms at least one of a wall, a ceiling and afloor of the test chamber. In some embodiments, the test chamber haswalls, ceiling and floor that are entirely reflective, with the variableabsorbing structure being placed there within. In some embodiments thestructure may have a rectangular cross-section. For example, thestructure may be a box having at least one side selectively exposingabsorbing material. In some embodiments, the structure has at least oneadjustable sliding shutter to selectively expose a portion of absorbingmaterial contained within the structure. The reflective material mayhave apertures that are selectively exposed by adjustment of a slidingshutter. The adjustable sliding shutter may be configured to roll into acylinder to conserve space. In some embodiments, the structure has atleast one adjustable louver to selectively expose a portion of absorbingmaterial contained within the structure. In some embodiments, thevariable absorbing structure includes a cylinder. The cylinder may havea rotatable portion to selectively expose a portion of absorbingmaterial contained within the cylinder. In some embodiments, thestructure exhibits individually variable apertures having sizes selectedto achieve a desired performance of the test chamber. Further, in someembodiments, the absorbing material may exhibit an absorptioncharacteristic that varies as a function of position along a length ofthe absorbing material. In such embodiments, the structure may beadjustable to selectively expose absorbing material in the directionalong the length of the absorbing material in which the absorptioncharacteristic varies. And, as noted above, the structure may beadjusted by remote control to vary the loading of the reverberationchamber. In some embodiments, the structure is adjustable so that allbut a selected range of angles of propagation in the test chamber aresuppressed. In some embodiments, the structure is adjustable toselectively absorb a particular polarization of energy in the testchamber.

Embodiments described herein further include a method for affectingperformance of a reverberation chamber. The performance is varied bypositioning a variable absorbing structure within the reverberationchamber at a predetermined position and orientation to achieve arepeat-ably achievable specific performance. The method may furtherinclude adjusting a movable portion of the variable absorbing structureto vary exposure of absorbing material of the variable absorbingstructure. The specific performance that may be varied may include powerdelay profile, RMS delay spread, Q factor, and K factor, and angulardistribution. In some embodiments, the specific performance is one of apolarization selectivity and a directivity.

Some embodiments described herein include a test chamber having anenclosed housing with at least one at least partially reflectiveinterior wall and an absorbing structure having an absorbing material.The variable absorbing structure is configured to selectively andrepeat-ably vary exposure of the absorbing material to achieve arepeat-ably achievable specific performance. The variable absorbingstructure may be remove-ably affixed to a surface of the housing or maybe one or more walls, ceiling and/or floor of the housing. In someembodiments, the variable absorbing structure is selectively variable toabsorb energy propagating within a first range of angles and to reflectenergy propagating within a second range of angles. In some embodiments,the variable absorbing structure is selectively positioned tosubstantially restrict rays of propagation falling within a selectedrange of angles. In some embodiments, the variable absorbing structureis selectively positioned to substantially suppress propagation above aspecified elevation. Some embodiments include a two dimensional stirreroriented to reflect energy between two orthogonal directions withoutsubstantially reflecting energy to a third orthogonal direction.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. In addition, unless mention was made above tothe contrary, it should be noted that all of the accompanying drawingsare not to scale. A variety of modifications and variations are possiblein light of the above teachings without departing from the scope andspirit of the invention, which is limited only by the following claims.

What is claimed is:
 1. A variable absorbing structure position-ablewithin a reverberation chamber and configured to provide absorption ofelectromagnetic energy in the reverberation chamber, the structurecomprising: reflective material configured to reflect electromagneticenergy; and electromagnetic absorbing material configured to absorbelectromagnetic energy, the electromagnetic absorbing material at leastpartially shielded by the reflective material, the reflective materialbeing adjustable relative to the absorbing material to expose a selectedportion of the electromagnetic absorbing material in order torepeat-ably and selectively control an electromagnetic fielddistribution within the reverberation chamber.
 2. The structure of claim1, wherein the structure forms at least one of a wall, a ceiling and afloor of the reverberation chamber.
 3. The structure of claim 2, whereinthe structure includes a box having at least one side selectivelyexposing electromagnetic absorbing material.
 4. The structure of claim1, wherein the structure has at least one adjustable sliding shutterhaving the reflective material to selectively expose a portion ofelectromagnetic absorbing material of the structure.
 5. The structure ofclaim 4, wherein the reflective material has a plurality of aperturesthat are selectively exposed by adjustment of at least one of the atleast one adjustable sliding shutter.
 6. The structure of claim 1,wherein the structure has at least one adjustable louver having thereflective material to selectively expose a portion of electromagneticabsorbing material of the structure.
 7. The structure of claim 1,wherein the structure is positioned to reduce an RMS delay spread in thetest chamber while substantially maintaining a uniform distributionwithin a test volume within the test chamber.
 8. The structure of claim1, wherein the structure includes a cylinder having the electromagneticabsorbing material.
 9. The structure of claim 1, wherein the absorbingmaterial exhibits an absorption characteristic that varies as a functionof position along a length of the absorbing material.
 10. The structureof claim 1, wherein the structure is adjustable so that all but aselected range of angles of propagation in the test chamber aresuppressed.
 11. The structure of claim 1, wherein the structure isadjustable to selectively absorb a particular polarization of energy inthe test chamber.